Portable Fuel Tanks: Sizing a 12, 18, or 24 Gallon Topside Tank

Fuel tanks portable enough to carry aboard when full top out at about 6 gallons; past that you are choosing a topside tank that gets strapped to the deck and plumbed like an installed tank. The decision is capacity, and capacity is set by three numbers: your motor's burn rate in gallons per hour, the hours you want to add, and what that fuel weighs once it is sitting on the casting deck or in the cockpit. Here is how the Moeller 12, 18, and 24 gallon topside tanks sort out on those numbers.

What portable fuel tanks mean past 7 gallons

ABYC H-25, the portable gasoline fuel system standard, stops at 7 gallons (26.5 L). Above that a tank is treated as installed, which in practice means it has to be secured so it cannot shift or tip, it gets a proper fuel hose and primer bulb rather than a tote's squeeze-and-go setup, and the vent has to be somewhere fumes cannot collect. Moeller's topside tanks are built for that role: high-density polyethylene with UV-stabilized resin, a mechanically vented fuel cap, a 90 degree 1/4 inch NPT withdrawal elbow, and molded provisions for tie-down straps. They live on deck, above the sheer, where a vented cap can breathe to open air. They are not below-deck tanks and should not be buried in a closed compartment.

Size it by burn rate, not by what fits

A four-stroke outboard at wide-open throttle burns roughly 1 gallon per hour for every 10 horsepower; at a 4,000 rpm cruise it is closer to half that. A carbureted two-stroke runs about 30 percent thirstier. So a 150 hp four-stroke cruising burns 8 to 10 gph, a 60 hp burns 3 to 4, and a 25 hp kicker at trolling speed burns under 1. Apply the rule of thirds, one third out, one third back, one third reserve, and the added range from each tank at cruise looks like this:

Those weights use 6.1 lb per gallon for gasoline and 7.1 lb per gallon for diesel. The point of the column is not trivia: 146 lb of fuel on a bow casting deck changes how an 18 ft bay boat rides, and 170 lb of diesel on one side of a cockpit lists a small sailboat noticeably. Put the tank on centerline when you can and as low as the deck allows.

12 gallon low profile: the kicker and the casting deck

The Moeller 12 Gallon Topside Fuel Tank with Low Profile is the one to buy when the tank has to sit under a gunwale, on a casting deck, or in a skiff where a tall tank gets kicked. Twelve gallons runs a 25 hp kicker for most of a day and adds three to four hours of cruise on a 60 hp tiller boat. The low profile also keeps the weight down where it belongs. Its trade is footprint: it is longer and wider for the same gallons, so measure the deck before you order.

18 gallon: the day-range tank for a center console

The Moeller 18 Gallon Topside Fuel Tank is the middle answer and the one most bay and center console owners land on. It adds two hours of cruise to a 150, which is the difference between turning back at the second reef and fishing the third. The molded tie-down channels line up with Moeller's hold-down kit, and the 1/4 inch NPT elbow takes a standard 3/8 inch hose barb for the feed line. On a boat with a 40 to 60 gallon built-in tank, 18 gallons topside is a sensible reserve; on a boat with no built-in tank it is a main.

24 gallon: range extension and transfer

The Moeller 24 Gallon Topside Fuel Tank is for long runs, for a diesel auxiliary that sips fuel, or as a transfer tank you fill at the fuel dock and pump into the main. Twenty-four gallons of gasoline is 146 lb before the tank, so this is a cockpit-sole tank, not a bow tank. Used as a transfer tank, a 12 V gasoline-rated or diesel-rated transfer pump moves it into the main in a few minutes; used as a running tank, the same elbow and hose setup as the 18 applies. One warning for transfer use: these are marine tanks. Hauling fuel on the highway in a pickup bed falls under DOT rules that vary by state and by fuel, so check before you use a topside tank that way.

Securing and plumbing the tank

Every one of these tanks should be strapped down with the Moeller Topside Fuel Tank Hold Down Kit, part 035711-10-1, which fits the molded tie-down provisions on the topside tanks. A 146 lb tank that slides in a following sea is a hazard before it is a leak. Mount the tank so the withdrawal elbow faces the motor, thread on a 1/4 inch NPT to 3/8 inch barb with fuel-rated sealant, run USCG Type A1-15 fuel hose through a primer bulb to the motor, and clamp both barbs with two stainless hose clamps each. Keep the vent cap clear; do not stack gear on it. Gasoline tanks should sit where a spill runs overboard, not into the bilge.

For diesel, the same tanks and hardware apply, with two differences. Diesel does not flash the way gasoline does, so the vapor concern is smaller, but diesel grows sludge in a tank that sits half full through a warm season, so either run it down and refill with fresh fuel or dose a biocide at the rate on the bottle. And diesel is heavier, so the 24 gallon tank full of diesel is a two-person lift onto the boat.

Safety, once: nitrile gloves and eye protection when plumbing fuel, no sparks or running engines while filling, and fill topside tanks on the dock rather than in the cockpit so vapors go over the side.

The recommendation

For a kicker, a tiller skiff, or any tank that has to go under a gunwale, buy the 12 gallon low profile. For a center console adding a day of range to a 90 to 200 hp outboard, buy the 18 gallon. For diesel auxiliaries, long offshore legs, or a transfer tank you fill at the fuel dock, buy the 24 gallon. Buy the hold-down kit with any of them; a topside tank without straps is not installed, it is loose cargo.

Frequently asked questions

What You Need For This Job

• Moeller 12 Gallon Topside Fuel Tank with Low Profile: kicker, skiff, and under-gunwale tank
• Moeller 18 Gallon Topside Fuel Tank: day-range reserve for a center console
• Moeller 24 Gallon Topside Fuel Tank: long runs, diesel auxiliaries, and transfer duty
• Moeller Topside Fuel Tank Hold Down Kit: straps any of the three to the deck